On Fri, 30 Dec 2005 16:52:43 GMT, Ignoramus24987
wrote:

I have a approximately 20x20 ft attached garage. Sometimes I want to
make it warm quickly. Often I am there for relatively short periods of
time (because I am constantly interrupted).

I bought a 9.6 kW, 240V duct heater on ebay. It has a 24V control and
various overheat protections.

My plan is to enclose it into a welded frame and sheet metal (found a
few nice sheets in a hospital dumpster), add a fan to it and a 24V
control system using a thermostat. I also have a grill type thing that
I can use to protect the coils from little curious fingers and
flammables.

My idea is to make something relatively compact (say 2x2x1.5 ft), that
I would set on the floor, turn on and get the garage warm in a few
minutes rather than wait for a long time. I would save some
electricity by only heating the garage when I need it, rather than
warming it up "just in case".

What I am looking for is some feedback on safety and such.

Make sure you test the overheat switches, and they kill the heater
power when tripped. A sail switch as a draft prover from the fan
before the heat coils can kick on would be a good thing to add - could
be rigged up with a spatula on a hinge pushing in a microswitch.

Unless you have a diaphragm pressure switch handy rated at a few
inches Water Column operating pressure. Connect the high side to the
heater plenum and leave the low side exposed to atmospheric.

Use a squirrel-cage blower, if you have a power failure they spin
down slowly enough to mostly cool off the heating coils. And put a
furnace filter on the inlet to trap the larger chunks, before you have
to smell them burning on the heater coils.

And hook up individual toggle switches to each separate heat coil
inside, so you can vary the load...

And when you get all done building this, set it aside and save it
for use as a 10KW generator load bank - and look for a Propane or
Natural Gas fired heater. ;-) You'll spend a WHOLE LOT less on fuel
than you will for the equivalent amount of electric resistance heat.

Myself, I have a little propane radiant heater on a 20-lb bottle.

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

Another side of Bruce's post:

If you don't move enough air through the duct heaters, you risk
overheating them and burning them out. (And you also risk fire and
excessive temps on the outlet). But if you try to move too much air,
you risk overheating the blower motor and burning it out.

If you use a furnace or AC blower to push air through your electric
duct heaters, and you don't have any ductwork to speak of on the
output, make sure to throttle the inlet side down. These centrifugal
blowers need some static resistance to stay on the proper portion of
their "fan curve". Otherwise, the blowers try to push so much air that
the blower motor overheats.

You need about 0.2 inch wc minimum static pressure across an HVAC
blower to stay out of the "overload" region. Usually, blocking off
about half of the inlet is sufficient to accomplish this. The suggested
inlet air filter will help in this endeavor as well.

A lot of guys have learned about this the hard way, when they try to
use a blower from a junked furnace to push air in the summer time.

You need to know how much air you are moving, not just "it feels like a
lot of wind"

Temp rise = BTU per hour / (CFM * 1.08)

You are releasing 33000 BTU per hour with your 9.6 KW duct heaters

Temp rise should be held to about 40 degrees or less for safety and
reliability of the duct heaters.

So you need at least 750 CFM through your duct heater. It needs to be
spread out across the full width of the heating coils.

Big muffin fans move about 75 to 100 CFM. So you'd need 8 or 10 of
them. If one of them fails, you risk localized hot spots in your
heating coils.

A high-end 20 inch box fan can move 2000 CFM with no static resistance.
Maybe 1500 CFM through your filter and ducting setup. So it would be a
better choice. If you use a garage sale fan, you should check the
outlet air temp to make sure it stays reasonable. Make sure to keep the
plastic blades well away from the duct heaters.

Large Dayton squirell cage blower on craigslist sandiego. Run it slow
and connect some kinda electric heater to the inlet side.
I still think just letting the car run in there with exhaust outside of
course will heat your space in under 15 minutes.